The study of airflow characteristics is an important part of basic aeronautical research and design. Air passing over a flight surface, such as a wing, is either laminar, transitional, or turbulent in nature. Air that flows smoothly in a continuous stream is laminar, while an air stream that is rough or broken is turbulent. Transitional airflow, as the name implies, alternates between laminar and turbulent conditions. As the airflow over an aircraft wing becomes turbulent, the fuel efficiency of the aircraft decreases. Therefore, in order to achieve maximum fuel efficiency of an aircraft, it is necessary that the airflow be laminar over as much of the wing surface as possible. Modem aircraft designers are currently experimenting with ways to alter the airflow patterns over an aircraft's wings while in flight in order to increase fuel efficiency. If these methods prove successful, pilots will be provided with controls or automated systems that alter the airflow over the wings to maintain maximum fuel efficiency in any given flight situation.
The most common way of measuring the characteristics of airflow over a wing is by using a hot film sensor as disclosed in U.S. Pat. No. 4,727,751. This method involves placing a plurality of hot film sensors at various locations on the wing and measuring the heat transfer from the sensors to the airstream as they are exposed to laminar, transitional or turbulent airflow. Such hot film sensors are coupled to an anemometer circuit, which drives them to a constant temperature with a feedback circuit such as that shown in FIG. 1 in the attached drawings or excites them with a constant current, depending on the experimenter's preference.
In the circuit shown in FIG. 1, an amplifier 10 drives a bridge network, including resistors R1, R2, R3, and R4. Resistor R4 is a hot film sensor having a resistance that varies with temperature. In operation, the feedback circuit supplies sufficient power to heat resistor R4 to a constant temperature so that the bridge network is balanced. As resistor R4 is cooled by the varying types of airflow passing over it, the feedback circuit adjusts the current supplied to reheat the resistor so that the bridge remains balanced. A capacitor 12 couples the output of the amplifier 10 to a signal measuring device, such as an oscilloscope, to determine the type of airflow flowing over the sensor. While the circuit in FIG. 1 works well for test conditions that take place at a relatively constant temperature, it has some serious limitations for determining airflow quality in varying ambient conditions.
In actual test conditions, the ambient temperature to which a wing is exposed may drop from 120.degree. F. at sea level to -60.degree. F. at 60,000 feet. The feedback circuit shown in FIG. 1 attempts to maintain the temperature of the hot film sensor constant, regardless of the ambient temperature of the air flowing over the wing. By requiring that the temperature of the sensor R4 be maintained at some specified level, the power-supplying capability of amplifier 10 can easily be exceeded. Furthermore, if the hot film sensor R4 is maintained at some relatively high temperature, such as 130.degree.-150.degree. F. for a 10.degree.-30.degree. temperature over ambient (overtemp) at sea level, this would result in a 190.degree.-210.degree. overtamp at -60.degree. F. At these temperatures the hot film sensor will tend to heat the air flowing over it, thereby altering the characteristics of the airflow that the circuit is trying to sense as well as burning the wires that supply heat to the sensors.
Another problem with the feedback circuit of FIG. 1 is that the resistance of the leads that extend from the hot film sensor R4 to the feedback circuit located in the cabin also varies with temperature. On a large airplane wing, the lead resistance may be of the same magnitude as the sensor. Because these leads are effectively included within the bridge circuit, the output signal of amplifier 10 cannot distinguish between a change in resistance of the sensor and a change in resistance of the leads. While not theoretically a problem, most implementations of the constant temperature feedback circuit of FIG. 1 will not faithfully follow the large variations in heat transfer when the sensor is subjected to transitional airflow conditions.
Another method of sensor excitation taught by prior art is the constant current method. With this method, a constant current is caused to flow through the hot film sensor at all times. However, since the sensors normally have a positive temperature coefficient, as the sensor is heated, the resistance rises thereby requiring more power to be delivered to the sensor to maintain the level of current. The increased power in turn heats the sensor, raising its resistance which requires more power etc. If the temperature of the sensor rises enough, it may be destroyed by overheating. Conversely, as the sensor is cooled, the excitation power required to maintain the constant current decreases, further cooling the sensor, etc. Thus, the sensitivity of the sensor varies with ambient temperature.
Finally, none of the prior art methods of sensing airflow conditions provide an output signal that allows a test engineer to quickly determine the type of airflow being sensed. The output signal of the prior art methods must be coupled to an oscilloscope or some other electronic test equipment that visually displays the signal to allow an interpretation.
In order to overcome the limitations of prior art airflow sensing systems, an airflow sensing circuit is required that is capable of operating over a wide range of temperatures. Similarly, a circuit is needed that is insensitive to variations in the resistance of the lead lines that extend from the sensor to the instrumentation within the aircraft. Furthermore, a circuit is desired that will follow the large dynamic range excursions of transitional airflow conditions. Finally, a sensing system is desired that provides a test engineer with an immediate indication of the type of airflow being sensed, without the need to interpret a visual waveform representation of the sensor signal.